Wafer processing requires cleanliness which to a large extent is obtained through automatic operator-controlled wafer handling machines. Typically such machines include a lifting platform and a pair of double-sided combs which are together rotatable so that the combs can pick up a given number of relatively widely-spaced spacers, and can interleave that group with another widely spaced group of wafers. The combs, after the wafer interleaving process is completed, rotate to a new comb position to accommodate all of the interleaved wafers. Interleaved wafers are placed into a quartz boat which is specially designed to hold the interleaved and closely spaced wafers for placement in a high temperature oven for processing.
Since the wafers are thin and fragile, it is important that they do not touch each other, or come into sharp contact with hard objects. Clean Teflon wafer cassettes, for example, may be loaded with about twenty-five wafers at a time. The machine interleaves these twenty-five wafers with another group of twenty-five so that all fifty wafers are closely spaced for loading into the quartz boat. If any wafer is not in its intended pair of opposed comb slots, there is a danger of wafer breakage.
Wafer interleaving involves two carriers or "combs" which space two distinct wafer groups for interleaving into a single larger group. A misaligned wafer can appear at any wafer slot location in either the moveable comb or the stationary comb. That misaligned wafer in one embodiment causes a mechanical movement in one of the carriers in order to activate an error signal that is rapidly interpreted by a computer driven interlacing machine and corrective action is taken. It has been discovered that the location of a misaligned wafer or foreign object creates complex movement problems which may result in breakage and damage of the fragile wafers. Moreover, the two groups of wafers during relative movements must be self-centering into longitudinally offset locations so that neither group touches the other or any solid part of the carriers or machine with sufficient force to cause breakage.
It has been known in the prior art to use a single light beam and a single electro-optical receiver moveably positioned on opposite sides of a plurality of parallel spaced wafers to scan the wafers. The beam's transmitter and receiver move together during scanning so that light passes between the wafers when they are properly spaced. If a wafer is skewed, then the transmitted light beam does not reach the receiver and an error is detected. Such a prior art technique requires scanning of every group and wastes time since the beam must physically move across the group to be scanned. Moreover, wafer misalignment or some foreign matter which is not detectable by the scanning process, can contribute to defects in wafer handling.
This invention improves upon the prior art by employing redundant electro-optical scanning and a plurality of error-initiated safety features.